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Re: [Phys-l] what to do with inherited magnets



On 11/28/2007 11:53 AM, kyle forinash wrote:

Through a long chain of circumstances our (very small; two people)
department ended up with two sets of very large electro magnets and very
old (25+ years) electronics intended to do ENDOR (Electron-Nuclear
DOuble Resonance) experiments. The person who contributed this equipment
got as far as getting water and 220V lines into the room and figuring
out the power supply for the magnet no longer functions before he go
himself fired (for other reasons). We have no grad students, do not have
even an undergrad major in physics (although we have a junior level lab
course) and I am pretty much all thumbs in the lab except for relatively
simple things.
My questions are:
1) Does anyone have any idea what kind of time and money resources would
be required to get this up and running for a student lab (or is this way
beyond what could be done in a junior lab even if it were working)?
2) What are my options here, short of hiring someone who knows what they
are doing and has a large budget to replace equipment? Are there simple
experiments that can be done with these large magnets (I'm not even sure
how big they are but the former owner said they cost on the order of
$40k each)?

Well, that's a problem. It's not a small problem, but
on the scale of things, it's a nice problem to have.

I suspect those are lovely magnets, and very valuable.

ENDOR is a combination of NMR and ESR ... and I don't
think you will be doing ENDOR in the junior lab anytime
soon. Start with NMR _or_ ESR. Or better yet, you can
start with qualitative things like eddy currents and
Lenz-law forces. In a magnet that size, you can get an
unforgettable hands-on feeling for the forces. There's
also Hall effect as WCM mentioned, and other solid-state
stuff that is particularly trendy given this years Nobel
prizes.

Non-physicists would just *buy* a working turn-key NMR
or ESR system. Physicists build 'em from scratch.

You will soon discover, if you haven't already, that the
magnets have a huge inductance and a very low resistance.
That is, the imaginary part of the impedance is huge and
the real part is small. This is "interesting" in the
Chinese sense. Most people get their first experience
working with electrical doo-dads like light bulbs that
use a relatively high voltage and low current, and are
not very reactive. The magnet uses a huge current at
a low voltage, and is very reactive.

The water cooling suggests that the things are rated
for something on the order of a thousand watts. What
you don't know until you measure it is whether that
is 100 volts at 10 amps (10 ohms) or 100 amps at 10
volts (10^-1 ohms). So the first thing to do is get
an order-of-magnitude measurement of the resistance.
That will determine whether you have any hope of driving
it with an ordinary "laboratory grade" power supply (as
opposed to a specialized magnet driver).

You can run the thing without water cooling if you
reduce the power limit accordingly.

If the thing is in the 4 to 10 ohm range, you might
be able to drive it with a hi-fi audio amplifier.
That falls into the real-quick real-dirty category,
but it might get you started.